This invention relates to iron oxide particles useful as magnetic recording materials. In particular the invention relates to iron oxides which are surface modified with cobalt and, at times, with other metal ions such as chromium and ferrous ions.
The literature is replete with materials which are surface modified with cobalt and oftentimes with other metals such as chromium. For example, the following U.S. patents discuss such materials: U.S. Pat. Nos. 4,066,564; 4,066,565; 4,069,164; and 4,071,654. There are also references to such magnetic materials in foreign patent applications. For exaple, Japanese applications Nos. 51-38094, 51-38095, 51-38096, 52-24237, 49475/74, 44040/73 and 87397/73 all make reference to such materials.
While the above named patents and applications represent a continuum in the advance of the art of magnetic materials which are surface modified, it is the present invention which provides a cobalt modified magnetic iron oxide for magnetic recording which has a minimal degradation of orientation ratio even at relatively high cobalt levels. Such degradation usually occurs when one increases the cobalt level in order to increase the coercivity (Hc). A further object of the present invention is to provide a cobalt modified magnetic iron oxide with yet another superior characteristic which we have termed M-factor. This characteristic is related to magnetostriction and it will be fully defined further in this application.